Aircraft that operate at high flying altitudes are normally equipped with a pressurized cabin that is accessible through a lockable door. During flight, the pressurized cabin ensures an essentially constant internal pressure in the cabin that is largely independent of the specific flying altitude and of the prevailing atmospheric ambient pressure and that is tolerable for the aircraft occupants, said internal pressure in the cabin corresponding approximately to an atmospheric air pressure at an altitude of about 8,000 feet to 10,000 feet at the maximum. Since the atmospheric ambient pressure drops as the flying altitude increases, elevated pressure (“overpressure”) vis-à-vis the atmospheric ambient pressure prevails in the pressurized cabin at flying altitudes of more than 10,000 feet. The internal pressure in the cabin is normally regulated by means of an internal-pressure regulating device as well as by the so-called flight management system of the aircraft. When the aircraft has once again parked on the ground following a flight, the internal pressure in the cabin should once again match the atmospheric ambient pressure to ensure that the door can be safely opened. In other words, no pressure differential should exist between the interior of the cabin and the outside environment. This pressure differential will be referred to below as the cabin differential pressure.
Since an internal cabin pressure that is lower than the atmospheric ambient pressure (that is to say, “underpressure”) is very dangerous for the structure of the aircraft, aircraft equipped with pressurized cabins usually have special protection mechanisms with one-way valves that prevent the build-up of a negative differential pressure (“underpressure”). In spite of this, after a flight, it can nevertheless happen in some cases that a certain cabin differential pressure is present. As a rule, this is an “overpressure” (although slight “underpressures” are also possible). If the door is unlocked and opened in this state, it flies open in an uncontrolled, quick manner, creating a strong air current that exerts a corresponding suction effect. This poses a considerable hazard to persons who are operating the door or who are in its immediate vicinity. Moreover, this can cause structural damage to the aircraft.
For this reason, aircraft that have a pressurized cabin are equipped with a cabin differential pressure warning system that, for example, detects the presence of any cabin differential pressure when the door is about to be opened and emits a warning signal. The aircraft crew can then first vent the pressurized cabin prior to opening the door, equalizing the pressure between the cabin and the outside environment, so that the door can then be opened without risk.
Conventional cabin differential pressure warning systems that indicate the presence of a cabin differential pressure, for instance, by means of a warning light, are connected to the electric energy supply network of the aircraft, which provides it with electric energy. If this electric energy supply network fails, the cabin differential pressure warning system no longer functions. Consequently, the personnel operating the door can no longer be warned about the cabin differential pressure, and the above-mentioned risks remain. A comparable hazard potential exists during maintenance work if the electric energy supply of the aircraft is not available and the cabin is under pressure.